While the invention is subject to a wide range of applications it is especially suited for use in air/ground radio telephone communications and will be particularly described in this connection.
In providing telephone service in aircraft, which may be used by passengers without any special radio operating experience, it is essential that the signal-to-noise ratio be at least commensurate with typical terrestrial telephone circuits the entire period of a normal length telephone conversation. For example, if the telephone conversation extends for a six minute interval on a high speed aircraft, say traveling at MACH 1 and subject to strong tail winds at 200 miles per hour, the aircraft can travel almost 100 terrestrial miles during the course of the conversation. If such systems use UHF channels and moderate transmitter power levels, the aircraft can fly out of the range of a specific ground station if the ground station is incorrectly selected. Therefore, it is essential, if reliable telephone communication is to be provided, that the selected ground station be "best" not only when the phone conversation is initiated but during an entire reasonable length conversation.
In the following, the term "best" signal refers to a signal that is expected to provide good performance, not only when a telephone call is initiated, but also for a period of time into the future. Thus, actually the selected "best" signal may not be the strongest signal or the signal enjoying the highest signal-to-noise ratio at the instant the selection is made.
A number of different performance criteria falls under this "best" designation, but common to all, is that part of the evaluation of which signal is "best", is whether a candidate's signal strength is expected to increase during the course of the telephone call.
Sophisticated voting methods for selecting one signal from a group of candidate signals according to signal strength or signal-to-noise ratio, are known, but such system selections are based upon the existing circuit conditions and do not consider future conditions. The instant invention treats both present and short term future circuit conditions.
It is also possible to implement selection of a ground station during the course of a flight by knowledge of the flight plan and the location of the various ground stations along the flight path. However, such a procedure requires pre-programming of circuitry and an adherence to a specific flight plan. Furthermore, weather, traffic conditions, etc. may require a change of flight plan and, accordingly, such a pre-programmed procedure will fail. It is noteworthy that a change in flight plan during the course of a trip would be a major reason for high activity of the air/ground telephone facilities. Thus, under a condition where there would, almost certainly, be a large amount of telephone traffic the system would be prone to fail.
Another possible technique would be to feed navigational information to the radio telephone communications equipment, allowing the navigational information to be used to locate the "best" ground station. Such a procedure would complicate the equipment and would increase costs. Furthermore, since the navigational equipment is of extreme importance to air safety, any failures caused by the interfacing of the telephone communications equipment would be most unacceptable.